1. Field of the Invention
The present invention is related to determination of volume and flow rate of a fluid through a prover and, more particularly, to precise detection of the position of the piston within a prover during translation of the piston within the cylinder.
2. Description of Related Prior Art
A purpose of a prover is to precisely determine the volume of fluid flowing therethrough. As a result, the measurements relating to such volume of fluid must be extremely precise. To achieve this end, fluid flows in one end of a cylinder, which cylinder includes a piston rectilinearly translatable from a location proximate one end of the cylinder to a location proximate the other end of the cylinder. The cylinder is generally precision machined and honed to provide the high level of sealing required for precision measurement. The piston may incorporate a flow through, or poppet type apparatus to permit bypass of the fluid after the measurement portion of the translation. After translation of the piston, the fluid flows out of the other end of the cylinder. The piston is supported upon a shaft extending through seals and bearings at each end of the cylinder.
Conventional small volume provers for calibrating a liquid flow meter with respect to the measurement of a fixed volume typically utilize two or more fixed position optical slot sensors located on the stationary frame of the small volume prover. A mechanical feature, or flag, rigidly mounted to the shaft of the piston passes through these optical slot sensors. The flag breaks the path between the light emitter and light sensor and triggers a signal that indicates the position of the piston. Calibrated volumes may be determined between the positions of any subset of two of the total set of optical slot sensors. The exact known traceable volume between any two such switches is determined through a gravimetric water draw process. Such small volume provers can measure the average piston velocity based on the amount of time (e.g., a time delta) between the signals from any two optical sensors. The time delta and the known volumes can provide the average flow rate on each pass. Unfortunately, such optical sensors are difficult to align and require very precise placement, which adds cost and complexity to the prover manufacture and installation. If such optical sensors are misaligned, the moving shaft mounted flag may hit and destroy the optical sensors.
The known, measured, traceable, or “proved” volume is repeated on every pass of the piston or “prove” by signaling pulses as the flag crosses the optical slot sensors. A common problem in such prior art small volume provers is that the position of the gate or slot is integral to each optical sensor switch, and therefore subject to move with the switch itself. A switch may need to be removed and replaced for a variety of reasons such as replacement due to failure, scheduled maintenance, troubleshooting, access to other components, and inspection. If a switch is removed or replaced with the same switch or a different switch, the position of the location gate or slot is likely to have changed. This may be due to the inability to place the switch back into exactly the same location within the housing, due to a change of the location of the slot housing of the switch, or both. The location of the switch can change without the switch having been removed by an external force such as the wires being pulled, the switch being hit or pressed by a force from thermal cycling or vibration over time. Any movement of the location of any of the gates or slots results in a potential change in the distance between them, which necessitates a recalibration of the known volumes. This is an expensive and time-consuming process.